package current_incr

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package current_incr
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A simple library for incremental computations. Based on "Adaptive Functional Programming" https://www.cs.cmu.edu/~guyb/papers/popl02.pdf

The basic idea is:

  • You run a computation, which produces a result.
  • The library records information about the dependencies of the result.
  • You can then change one or more inputs and it will rerun just the parts of the computation that are needed to update the result.

It is similar to Jane Street's "incremental" library, but much smaller and has no external dependencies.

It is also similar to "react", but the results do not depend on the behaviour of the garbage collector. In particular, functions stop being called as soon as they are no longer needed, and cannot be called with "impossible" inputs.

Changeable values

type !'a t

An 'a t holds a value of type 'a, which will update as necessary.

val const : 'a -> 'a t

const x is the constant value x.

Changeable computations

type !'a cc

An 'a cc is a changeable computation that can be run to get a value of type 'a. Internally, it is a function that takes a destination variable, reads zero or more other changeable values, and then updates the destination.

val read : 'a t -> ('a -> 'b cc) -> 'b cc

read x f is a computation that depends on x. f will be called as necessary to ensure that the result stays up-to-date with changes in x.

val write : ?eq:('a -> 'a -> bool) -> 'a -> 'a cc

write x is a computation that always returns x.

  • parameter eq

    If eq old_value x = true then the new value is considered equal and no update is done. The default is (==). It is always safe (though inefficient) to return false here.

val of_cc : 'a cc -> 'a t

of_cc x is a changeable value holding the result of evaluating x.

val on_release : (unit -> unit) -> unit

on_release fn calls fn if the current computation has to be redone. This may be useful to release external resources when they are no longer needed. Note that the order in which multiple such functions are called is somewhat unpredictable.

val map : ?eq:('b -> 'b -> bool) -> ('a -> 'b) -> 'a t -> 'b t

A convenience function to read a value, apply a function to it, and write the result.

module Separate (Map : Stdlib.Map.S) : sig ... end

Processing each item of a set efficiently.

External operations

These functions are used to interface between the changeable system and other systems (e.g. Lwt).

type !'a var

A mutable value that can be changed using change. Internally, type 'a var = 'a t, but it's useful to distinguish between values that are inputs to the system and values that only change in response to computations being rerun.

val var : 'a -> 'a var

var x creates a new variable initially set to x.

val of_var : 'a var -> 'a t

of_var x casts x to 'a t.

val change : ?eq:('a -> 'a -> bool) -> 'a var -> 'a -> unit

change x v sets the current value of x to v and adds anything that depends on it to the rebuild queue. You must call propagate after this to update everything (you can change several things together and then call propagate once). This function can only be used from the top-level, not from within a computation.

val propagate : unit -> unit

Apply all changes made with change, so that everything is up-to-date.

val observe : 'a t -> 'a

observe t is the current value of t. If change has been used then the value may be stale until propagate is called.